UK Load in Amps on a mains single-phase circuit

[Carl Haworth]

I have a 230 Volt single-phase circuit with a 100 Amp cutout fuse and a 100 Amp MCB switch, so the supply is rated at 23 kVA.

Only about 13% of my total energy consumption is electricity. The remaining 87% is natural gas, supplying the following appliances (kW figures are net output power):-

22 kW boiler, 3 kW fan-flued convector heater. 8 Kw cooker.

After (if?) natural gas is turned off some time in the 2030s I can have 3-phase installed (at vast cost) to give me total kVA 55.15 at 230 Volts. As you can see, this offer is tied to reducing the supply capacity per phase to 80 Amps. So a connection from one phase (and the neutral) to my existing single-phase consumer unit would be rated at 18.40 kVA, leaving (I presume) 36.60 kVA for the 3-phase, once this is balanced (how is balancing achieved in this sort of situation, by the way?).

I will also, by then, need a powerful EV charging wallbox. An 11 kW one is for 3-phase only, as would be a 22 kW electric boiler (heat pump technology is not suitable for our home). If these two appliances consume 33 kVA, the 3-phase will be used to over 80% of its capacity, but it could be arranged that the two would never be on at the same time.

The 23 kVA single-phase circuit, with capacity reduced to 18.4 kVA, will need to carry extra loads:-

Cooker (12 kW maximum, 7 kW diversity), 3 kW convector heater.

I have done calculations of the peak consumption of the circuit as it stands now. I need to be confident that these are reasonably realistic, so that I can add the extra loads to the existing ones, and calculate the much higher peak consumptions with the extra loads added. I need to know if the total is likely to be within the reduced 18.40 kVA capacity of this circuit.

If this is relevant (please see below), the additional appliances mentioned for the single-phase, and both those for the 3-phase, have power factor 1.

As regards the existing load on the single-phase circuit, if a p.f. should be applied, it should be reasonable to use 0.8.

To work out first the load on the increased-load circuit as regards the its capacity/the capacity of the cutout fuse, I need to know if I should increase the kW of the total existing load by 1/0.8, so as to include apparent as well as real current, and get a figure in kVA. Or should use the total kW. At that point I would add the extra loads of the cooker and convector heater, as kVA and kW are identical.

Calculating Amps using kVA increases the load on the existing circuit by 25% if p.f. is 0.8, so, after adding the extra proposed loads, there is unsurprisingly considerably more "headroom" left in the circuit capacity if I calculate using kW.

I suspect that kW is the correct basis to use, because apparent current passes back into the grid after passing through the circuitry of an appliance. An appliance may draw, say. 10 Amps. If its p.f. is 0.8, 2 Amps are returned to the grid instantaneously as the 10 Amps are drawn. This is why a Watt/hour meter records using the voltage times the net current used up by loads, and so reads in kWh and not kVAh.

So I suspect that the cutout fuse draws the net current (full current less apparent current).

Which should I use for my calculation: kW or kVA? If the answer is kVA, then, in view of my comments just above, where have I misunderstood things?

I've tried to find an answer to this, but can't find any focussed guidance.

Can forum members enlighten me reliably, please?

Carl

 

[Avo Mk8]

I'm not able to answer your question I'm afraid, but have a comment or two:

You mention assuming a PF of 0.8 in your calculations.
I think it's most unlikely the PF would be as low as 0.8. Reports on the interweb (therefore must be true ) suggest a typical UK domestic figure for a household of around 0.95.

For a modest outlay you could purchase a meter to find the current position (sorry!) - just an example below:
(there are many cheap direct reading displays, but most seem to have a current transformer that is a rigid toroid, so cannot be placed on incoming tail - maybe through design!)

PEACEFAIR 100A Multi-function Meter Voltage Current Power Factor Frequency Energy Tester physical parameters(1*Meter+ 1*open type CT) : Amazon.co.uk: DIY & Tools

PEACEFAIR 100A Multi-function Meter Voltage Current Power Factor Frequency Energy Tester physical parameters(1*Meter+ 1*open type CT) : Amazon.co.uk: DIY & Tools

www.amazon.co.uk

AC LCD Digital Battery Multimeter Ammeter Voltmeter 80-260V LCD Display 100A UK | eBay

Monitor AC circuits more comprehensively! One interface displays 6 kinds of measurement parameters at the same time, including voltage, current, power, energy, frequency, and power factors, and display the data through the LCD screen. LCD HD display, 360° viewing without dead angle, with...

www.ebay.co.uk

If you have a smart meter, depending on the model, you may find sequencing through the displays shows not only the usual kWh, but kVArh as well, in which case you could work out PF from that.

 

[Carl Haworth]

I'm not able to answer your question I'm afraid, but have a comment or two:

You mention assuming a PF of 0.8 in your calculations.
I think it's most unlikely the PF would be as low as 0.8. Reports on the interweb (therefore must be true ) suggest a typical UK domestic figure for a household of around 0.95.

For a modest outlay you could purchase a meter to find the current position (sorry!) - just an example below:
(there are many cheap direct reading displays, but most seem to have a current transformer that is a rigid toroid, so cannot be placed on incoming tail - maybe through design!)

PEACEFAIR 100A Multi-function Meter Voltage Current Power Factor Frequency Energy Tester physical parameters(1*Meter+ 1*open type CT) : Amazon.co.uk: DIY & Tools

PEACEFAIR 100A Multi-function Meter Voltage Current Power Factor Frequency Energy Tester physical parameters(1*Meter+ 1*open type CT) : Amazon.co.uk: DIY & Tools

www.amazon.co.uk

AC LCD Digital Battery Multimeter Ammeter Voltmeter 80-260V LCD Display 100A UK | eBay

Monitor AC circuits more comprehensively! One interface displays 6 kinds of measurement parameters at the same time, including voltage, current, power, energy, frequency, and power factors, and display the data through the LCD screen. LCD HD display, 360° viewing without dead angle, with...

www.ebay.co.uk

If you have a smart meter, depending on the model, you may find sequencing through the displays shows not only the usual kWh, but kVArh as well, in which case you could work out PF from that.

Thanks!

I am puzzled that it is proving so difficult to establish whether the current load on a circuit, hence on its cut-out fuse, is the full current drawn (real plus apparent current) or the net current (real current only).

However, your suggestion of 0.95 p.f. for a typical domestic installation is not only less pessimistic than my 0.80, but may be more realistic. If the latter, it makes it less critically important to know whether I should be working in kW or kVA!

I will ask Octopus Energy about the Smart meter that they say they want me to agree to have. I would normally resist this to the wire (sorry!), because I fear large and disruptive implications (eg, meter position, and so on) for an installation like ours. I suppose(!) that we wouldn't have to meet the cost of replacing the supply over a distance of around 8 metres under our drive in order to bring it to a new, external meter box, but we would be left with a load of fall-out as regards "making good".

It would be crazy to get involved in this unless we were installing 3-phase at the same time. And we're certainly not ready to make a decision on this at the moment. In any case, if our DNO means what they say, our supply amperage would be reduced to 80 Amps per phase, rather than the present 100 Amps of our single-phase, so we'd be obliged to have whatever form of electrically powered heating we might decide on (when we are ready!) on 3-phase. New large loads would be needed for the single phase, which would be at (safe level) 80% of 80% the present capacity (64 Amps).

CURRENT MONITOR

Many thanks for the links, which I will follow.

I'm already investigating a monitor: Omega OM-DCEV. This uses two clamps on the phase wire:-

https://uk.farnell.com/omega/om-dvcv/data-logger-ac-voltage-current/dp/3410288?gclid=EAIaIQobChMIrJaL9uTP9QIV2MLVCh22wQyYEAQYAiABEgIMBfD_BwE&mckv=_dc|pcrid|565467377247|plid||kword||match||slid||product|3410288|pgrid|125527087130|ptaid|pla-301834726710|&CMP=KNC-GUK-GEN-SHOPPING-SMART-EDMIUMROAS-Test821&gross_price=true#anchorTechnicalDOCS

Do you have any knowledge of this monitor?

I've emailed Omega (whose price is £265 (they are - or appear to be! - the manufacturer) to try to establish points such as:

- Will it record total current or only p.f. current?

_ With a PSU replacing its AAA batteries, has it enough memory to log current over a year, or will it have to removed at intervals (what intervals?) to transfer the data collected to date to my PC, and then be replaced and re-started.

I'm hoping for an answer! (I'm sure you know the feeling!)

Best wishes,

Carl





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[plugsandsparks]

Fuse responds to the VA load, real and apparent

 

[Carl Haworth]

Fuse responds to the VA load, real and apparent

 

[Carl Haworth]

Thanks very much - as I feared!

However, AVO 8 MKO (hope i've got that right) considers that 0.95 is a more realistic p.f. for a domestic installation than 0.60, so the difference between the two narrows considerably!

Whew!

 

[Avo Mk8]

I am puzzled that it is proving so difficult to establish whether the current load on a circuit, hence on its cut-out fuse, is the full current drawn (real plus apparent current) or the net current (real current only).

Stating the obvious - the 100A fuse is there for protection against sustained overload, or some major mishap. Design standards require it to withstand a load of 1.3 x rated current indefinitely. 1.6 x rated current for 90 minutes. You've got to go some to blow it. I feel the power factor considerations you are concerned about are really neither here or there in the grand scheme of things.

I'm already investigating a monitor: Omega OM-DCEV. This uses two clamps on the phase wire:-
Do you have any knowledge of this monitor?

It seems to clip onto Line and Neutral, and use a current transformer.
I'm afraid I'm not familiar with it.
The Omega unit does not appear to monitor power factor, which I thought you were interested in?

The data logger I put a link to is under £22, connects to a PC via USB (with the right model), doesn't use batteries, and it would be the PC that records the data. But probably a lot less user friendly than the Omega!

I've emailed Omega (whose price is £265 (they are - or appear to be! - the manufacturer) to try to establish points such as:
- Will it record total current or only p.f. current?

I hope you get a prompt response.

The second link I posted is a gadget that can be set up to give a real-time display of your overall consumption, including power factor. I though it could answer your concerns about the PF value at your property. It is only £18.

Best regards

 

[Lucien Nunes]

I don't think anybody has adequately answered the original question. The maximum current rating of a conductor or fuse is determined by the heat dissipation within it, which depends on the square of the current through it. There is no relationship between the cable / fuse dissipation and the supply voltage, nor to the phase relationship between current and voltage.

Therefore when considering load power, the fuse / circuit size relates only to the apparent power in kVA and the symbols embody this concept. Apparent power in VA is simply the scalar product of the current and voltage (V and A) without any regard to their relative phase. It is the power that 'appears' to be flowing as observed by means of a voltmeter and ammeter alone.

100A at 230V will always transfer 23kVA because 230 * 100 = 23,000. If the power factor is unity that 23kVA will deliver 23kW of real power; if the pf is zero it will transfer 0kW. But the cables and fuses care nothing for the difference, as their dissipation depends only on the current which is 100A in both cases.

 

[Carl Haworth]

I don't think anybody has adequately answered the original question. The maximum current rating of a conductor or fuse is determined by the heat dissipation within it, which depends on the square of the current through it. There is no relationship between the cable / fuse dissipation and the supply voltage, nor to the phase relationship between current and voltage.

Therefore when considering load power, the fuse / circuit size relates only to the apparent power in kVA and the symbols embody this concept. Apparent power in VA is simply the scalar product of the current and voltage (V and A) without any regard to their relative phase. It is the power that 'appears' to be flowing as observed by means of a voltmeter and ammeter alone.

100A at 230V will always transfer 23kVA because 230 * 100 = 23,000. If the power factor is unity that 23kVA will deliver 23kW of real power; if the pf is zero it will transfer 0kW. But the cables and fuses care nothing for the difference, as their dissipation depends only on the current which is 100A in both cases.

Thanks, Lucien,

Where p.f. is less than 1.0 I still don't understand why the apparent current is part of the overall load on the main fuse. Obviously this current is passing through the fuse to the appliance which is drawing it.

Instantaneously. however, the apparent part of the current flowing through the appliance for whatever reason is - in addition to the real (p.f.) current, but not to perform work - returned to the grid via the fuse.

The returning apparent current cancels out its share of the total current load, so the Watt/hour meter "sees" real (working) current only (unfair on the grid, but nice for consumers).

Is the following a correct layman's understanding of why apparent current is part of VA from the point of view of the supply capacity/main fuse?

The current part of the VA supplied includes both the "arriving" and the lower "returning" current. Although the "returning" current cancels out its own potential share of the power developed by the appliance (Watts), because it does not do any work, it is still loading the fuse on its "way out".

So the total load on the fuse is still the total of the two classes (phass?) of current.

However, I don't understand your comment that an ammeter responds to the combination of real and apparent current. If that were so, surely the current coil of the Watt/hour meter would be developing a magnetic field on the basis of the combination of real and apparent current, so recording VA and not Watts?

In reality, surely, the "negative" effect of the returning apparent current cancels out the relevant part of the total current, so the ammeter reads only in "real" current Amps?

Obviously that is not the case with the load as such on the supply.

 

[pc1966]

Is the following a correct layman's understanding of why apparent current is part of VA from the point of view of the supply capacity/main fuse?

The fuse reacts to the heating effect of I^2R and are not considering if that I is in phase with the supply V (for real power) or at 90 deg to V (for the imaginary power).

What is actually used to deliver useful work is the real part. The imaginary part simply contributes to heating of the supply cables, etc, which is why large industrial users often have capacitor banks to compensate for this (brining PF closer to 1) as the supply company charge them otherwise, basically to keep their network operating efficiently.

 

[Carl Haworth]

Stating the obvious - the 100A fuse is there for protection against sustained overload, or some major mishap. Design standards require it to withstand a load of 1.3 x rated current indefinitely. 1.6 x rated current for 90 minutes. You've got to go some to blow it. I feel the power factor considerations you are concerned about are really neither here or there in the grand scheme of things.

View attachment 94374


It seems to clip onto Line and Neutral, and use a current transformer.
I'm afraid I'm not familiar with it.
The Omega unit does not appear to monitor power factor, which I thought you were interested in?

The data logger I put a link to is under £22, connects to a PC via USB (with the right model), doesn't use batteries, and it would be the PC that records the data. But probably a lot less user friendly than the Omega!

I hope you get a prompt response.

The second link I posted is a gadget that can be set up to give a real-time display of your overall consumption, including power factor. I though it could answer your concerns about the PF value at your property. It is only £18.

Best regards

I will now look at the links which you so kindly supplied, because the Omega continuous current monitor which I was looking at is not suitable for my needs (it has a memory capacity equal, at the highest sampling rate (once per second), to only about 9 hours' connection time, after which it switches off, and, as regards battery life, it can't, as it stands, be run from a mains-powered UPS,

And, it appears, it does not have CE approval so can't legally be sold in the UK - until there is a post-Brexit British Standard drawn up for the compliance to which it can be approved.

I was surprised by this, because, as with so much which was supposed to be part of "our own sovereignty" post-Brexit, I was under the impression that the UK was still recognising CE compliance, at least for devices which already have it.

I did have to ring Omega to discuss this monitor, but the technical man, Alex, was so pleasant and (within reason) helpful, that I did not grumble about not having had a reply to my message.

 

[Avo Mk8]

it does not have CE approval so can't legally be sold in the UK

It is possible the device does not fall under any Directives that require it to be CE marked.
It is battery powered (under 50V), so does not come under the LV Directive
It may be regarded as electromagnetically 'benign' so would not come under the EMC Directive,
Alternatively, like many electronic sub-assemblies, it may be for incorporation in an overall system, and thus be outside the scope of the CE Marking Directive itself.

With regard to post-Brexit arrangements, the process is in hand.


If UK manufacturers want to sell in Europe, they will still have to comply with EU CE marking procedures.
To sell in the UK they will from 2023 comply with UKCA marking procedure.
Until 2023, both schemes are valid in UK

 

[Lucien Nunes]

Is the following a correct layman's understanding of why apparent current is part of VA from the point of view of the supply capacity/main fuse?

No. First, there is no such thing as apparent current. The current is the current, There is apparent power which is the figure obtained by multiplying the current by the voltage and which makes up the full VA figure (not part of it.) Apparent power can be resolved into two parts for the purpose of analysis; real power delivered to the load, and reactive power which flows from source to load and back again every AC cycle. Reactive power flow to and from the source 'cancels out' because the current is out of phase with the voltage, but the current does not itself cancel out.

Any current that flows in the conductor, regardless of which way it is delivering power or how much of the power is real, causes voltage drop along the conductor that is in phase with the current. The phasing of the load voltage, which governs the real power, is unrelated.

 

[Carl Haworth]

No. First, there is no such thing as apparent current. The current is the current, There is apparent power which is the figure obtained by multiplying the current by the voltage and which makes up the full VA figure (not part of it.) Apparent power can be resolved into two parts for the purpose of analysis; real power delivered to the load, and reactive power which flows from source to load and back again every AC cycle. Reactive power flow to and from the source 'cancels out' because the current is out of phase with the voltage, but the current does not itself cancel out.

Any current that flows in the conductor, regardless of which way it is delivering power or how much of the power is real, causes voltage drop along the conductor that is in phase with the current. The phasing of the load voltage, which governs the real power, is unrelated.

That is a beautifully lucid (no pun intended!) explanation - and - indeed, I should not have used the term "current" with "apparent".

Now please excuse the following uneducated further questions, and suggested answers (I prepare to be shot down again!):-

But how is that the Watt/hour meter (I mean the meter which supplies data for consumption charging purposes to the energy supplier), is driven, at the supply voltage and frequency, by only real power?

Is it because the current which is the basis of the apparent power is out-of-phase with the current basis of the real power?

If so, why does the power developed by the out-of-phase current not do work in the meter's current coils?

Is this because the work done to drive the meter by the total current is reduced by the effect of the out-of-phase current which lags by 90 degrees behind the total current?

 

[Carl Haworth]

It is possible the device does not fall under any Directives that require it to be CE marked.
It is battery powered (under 50V), so does not come under the LV Directive
It may be regarded as electromagnetically 'benign' so would not come under the EMC Directive,
Alternatively, like many electronic sub-assemblies, it may be for incorporation in an overall system, and thus be outside the scope of the CE Marking Directive itself.

With regard to post-Brexit arrangements, the process is in hand.
View attachment 94410

If UK manufacturers want to sell in Europe, they will still have to comply with EU CE marking procedures.
To sell in the UK they will from 2023 comply with UKCA marking procedure.
Until 2023, both schemes are valid in UK

 

[Carl Haworth]

No. First, there is no such thing as apparent current. The current is the current, There is apparent power which is the figure obtained by multiplying the current by the voltage and which makes up the full VA figure (not part of it.) Apparent power can be resolved into two parts for the purpose of analysis; real power delivered to the load, and reactive power which flows from source to load and back again every AC cycle. Reactive power flow to and from the source 'cancels out' because the current is out of phase with the voltage, but the current does not itself cancel out.

Any current that flows in the conductor, regardless of which way it is delivering power or how much of the power is real, causes voltage drop along the conductor that is in phase with the current. The phasing of the load voltage, which governs the real power, is unrelated.

That is a beautifully lucid (no pun intended!) explanation - and - indeed, I should not have used the term "current" with "apparent".

Now please excuse the following uneducated further questions, and suggested answers (I prepare to be shot down again!):-

But how is that the Watt/hour meter (I mean the meter which supplies data for consumption charging purposes to the energy supplier), is driven, at the supply voltage and frequency, by only real power?

Is it because the current which is the basis of the apparent power is out-of-phase with the current basis of the real power?

If so, why does the power developed by the out-of-phase current not do work in the meter's current coils?

Is t

It is possible the device does not fall under any Directives that require it to be CE marked.
It is battery powered (under 50V), so does not come under the LV Directive
It may be regarded as electromagnetically 'benign' so would not come under the EMC Directive,
Alternatively, like many electronic sub-assemblies, it may be for incorporation in an overall system, and thus be outside the scope of the CE Marking Directive itself.

With regard to post-Brexit arrangements, the process is in hand.
View attachment 94410

If UK manufacturers want to sell in Europe, they will still have to comply with EU CE marking procedures.
To sell in the UK they will from 2023 comply with UKCA marking procedure.
Until 2023, both schemes are valid in UK

I knew vaguely about the above, so your neat diagram is useful. Many thanks!

Do I understand you correctly that CE approval is probably irrelevant to this device, anyway?

I'm merely curious about this, because the monitor is apparently on sale (at both Omega and Farnell), even though it seems not be what I should be buying!

The phone connection with the guy a Omega was not good, so he might have been referring to a different monitor, with a larger sampling memory, which he mentioned as one that they could not currently sell in the UK (or in the EU??).

But I pressed him on this block, and he seemed to be sure that it applied to the OM -DCVC. Seems odd, when it is for sale.

I've followed your two links. As far as I can tell, they show instantaneous data only, and do not display current etc over time in graphical form. Apologies if I have not understood correctly.

My energy company's website has some wordy (but not very worthy!) pages on its smart meter, but betrays little of of what this can and can't do beyond what most informed people already know. I'm reluctant to ask for full technical details because (a) the customer service staff seem to know only about supplying and charging for electricity, and (b) I don't want to unleash the remotest risk of my home's frontage being excavated to fit a new single-phase supply, probably with a lower amperage main fuse., all to leave me with a downgraded version of my existing supply, and a meter which does not provide a key thing that I want - only to have to repeat the exercise in a few years' time to install 3-phase!

in any case, it seems highly unlikely that a smart meter would have a memory to hold several months' record of current consumption rates and which I could transfer to, and keep in, my PC

I suspect that the question "how much spare capacity is there in my supply at recorded peak instants" is going to be asked with increasing frequency if energy consumers want, or conclude that they are going to be obliged, to switch to all-electricity from gas. Ideas floated by our government about first mixing 20% H2 with NG and eventually moving to all-H2 may prove impossible to realize for cost reasons (eg, replacing mild steel pipelines with ones of a steel resistant to H2, and/or no prospect of being able to manufacture enough "green" H2, and so on). And the net-zero date, to which the UK has signed up, looms!

So I suspect that interest will soonbe awakened fairly widely - among suppliers of electrically-powered heat-generating equipment, if not consumers - in being able to work out how much extra load a given single-phase installation can safely stand, and whether an expensive upgrade to 3-phase is needed (and if this would be enough).